Federal biologists have confirmed that ancient sturgeon not only exist in the San Joaquin River, but also spawn there, despite less than ideal conditions.

For years, evidence that the huge fish are swimming upstream past Stockton has been mounting, but it was all anecdotal.

Fishermen occasionally reported to the state Department of Fish and Wildlife that they'd snagged a sturgeon. One stretch of the San Joaquin near the Stanislaus River was nicknamed "Sturgeon Bend."

The fish were apparently numerous enough to attract poachers, seeking eggs that could be processed into valuable caviar. And there are legends that have proved difficult to confirm - like the time a team of horses supposedly hauled a monster sturgeon from the stream.

Biologists can now say with certainty the fish are there, not only in the San Joaquin, but also in tributaries like the Stanislaus River.

Experts still have much to learn. Why are the fish swimming up the San Joaquin when more desirable habitat is available on the Sacramento River, to the north? How viable are their offspring in a polluted river whose flow has mostly been diverted to upstream farms?

Biologist Zac Jackson, who works in the Stockton office of the U.S. Fish and Wildlife Service, began studying the issue in 2011. He anchored 3-foot-long mats in the river with the hope of collecting sturgeon eggs as they floated past.

It took just a week or so for those eggs to turn up.

"I was fairly confident we'd find them," Jackson said. "But I thought it would take longer."

The quick discovery may have been a matter of luck, "But I do think there's definitely a lot more going on than we are able to observe, just because of the enormity of the area," he said.

Twenty-three eggs were found in 2011 and 65 eggs were found in 2012, suggesting that sturgeon were spawning not only in wet years, when conditions were more favorable, but also in dry years.

Biologists also used nets to trap adult sturgeon and outfit them with tracking devices. Twenty-eight adults have been captured and released, measuring anywhere from 44 inches to 72 inches.

Tracking the fish helped biologists verify that the sturgeon also wander into tributaries on occasion, including the Stanislaus River. Jackson said he's "fairly confident" spawning is also occurring in those tributaries.

Sturgeon are bottom-feeders with long, pointed snouts useful for shoveling up food. They live mostly in the ocean or San Francisco Bay, traveling farther inland to spawn.

They can be enormous. In his well-known book, "Inland Fishes of California," expert Peter Moyle wrote the verified record for a white sturgeon is 1,300 pounds, one-third the weight of an average sedan. They have been known to live well into their 80s and beyond.

Moyle wrote that while most sturgeon spawning occurs on the Sacramento and Feather rivers, the San Joaquin might once have been an important spawning stream before its flows were reduced.

Sturgeon prefer cobbly, rocky river bottoms for spawning, along with cold and deep water, Jackson said. So it's a bit of a mystery how they've managed to propagate in the sandy, muddy San Joaquin.

The good news is that 70 percent of the eggs collected so far seem to have been "viable," Jackson said. The next step will be to study the fish at a larval stage to see how many offspring might be surviving toward adulthood.

Only then can biologists begin to understand whether the fish from the San Joaquin are numerous enough to contribute significantly to the Central Valley's overall sturgeon population.

The findings so far confirm the existence and spawning only of white sturgeon, the more common California variety. Green sturgeon are a threatened species; while fishermen have reported catching them on the San Joaquin, there remains no firm data on their existence upstream of Stockton.

"We're definitely looking for them," Jackson said.

He said it's encouraging that sturgeon have been able to spawn in the San Joaquin as it exists today. Plans to restore the river with additional flow might help even more in the future, he said, and ongoing work to improve habitat for salmon - such as pouring gravel into streams - might also benefit sturgeon, though this remains unclear.

"Working with sturgeon is definitely rewarding, and I'd like to do what I can to make sure they're around awhile longer," Jackson said.

Longtime Stockton fishing guide Jay Sorensen said it's no secret the sturgeon swim right through the city on their way up the river. He's skeptical that the San Joaquin population is great enough to make much difference though.

One thing's for sure: Fishermen love a good sturgeon fight. Sorensen remembers hooking a white sturgeon off Decker Island more than a decade ago. He handed the rod off to a 12-year-old boy, thinking the line might be caught on a snag.

Then the fish moved. Sorensen reached out and grabbed the boy, who was about to tumble overboard.

Five and a half hours later, Sorensen landed the 9.5-foot beast, estimated at 450 pounds and 90 years to 110 years of age. She was released back into the water, and might still be swimming around the Delta today - who knows, perhaps exploring the San Joaquin River.

The fish was so big, Sorensen said, "I could have dropped a football into its mouth."